The present invention relates to an improvement in a revolutionary new type of bicycle disclosed and claimed in U.S. patent application Ser. No. 266,181, filed May 22, 1981, now U.S. Pat. No. 4,669,747. Conventional bicycles comprise a rigid frame including a central post in which a seat is mounted, a top crossbar rigidly connected to the central post near the top thereof and extending forwardly to a front journal tube in which the separate front wheel frame or fork is rotatably journaled, and a bottom crossbar which extends upwardly and forwardly from the bottom of the center post, usually from the crank shaft hub mounted at the bottom of the central post, to the front journal hub at a point just below the top crossbar. The center post, top crossbar and bottom crossbar define the three legs of a rigid structural triangle.
In the revolutionary new bicycle of U.S. Pat. No. 4,669,747 the foregoing rigid, triangular-shaped frame member is eliminated. The bottom crossbar is eliminated altogether and provisions for a flexible spring connection between the top crossbar and the center post are made. One or more tension cables extend from the front portion of the frame, near the front of the top crossbar, downwardly to a point on the rear portion of the frame near the bottom of the center post. This construction is especially well adapted to dirt bike racing in that the frame itself is designed to absorb the shocks of rough riding. Further, when pedalling torque spreads the frame, the springs at the tension cable ends are compressed. At the end of the pedal stroke, the spring relaxes and drives the crank upward for an extra kick when the pedals are on the upstroke part of their travel.
A problem with this construction was that the front portion of the frame and the front wheel tend to yaw laterally from side to side relative to the rear frame portion as the bicycler pedals. This front yaw problem was solved by including stiffening means integral with the bicycle center post and/or the crossbars as disclosed and claimed in U.S. patent application Ser. No. 54,270, filed May 26, 1987, now U.S. Pat. No. 4,792,150. The stiffening means for the crossbar included a pair of vertically spaced crossbar members and also a flat rigid plate positioned inside each crossbar member and extending a major portion of the length thereof and oriented laterally with respect to longitudinal axis of the frame. In another embodiment, the stiffening means of a crossbar comprised a structure constructed of substantially flat consecutively connected wall portions formed into a box beam.
One problem with the crossbar stiffening means disclosed was that they involved the use of multiple parts which must not only be fitted and assembled with care, but also which add unnecessary weight. Also, with the additional parts, tolerances became critical and manufacturing complexity was increased.